Schistosomiasis

Schistosomiasis is a parasitic infection caused by a type of parasitic flatworms (flukes) belonging to the genus Schistosoma.

It is the third most deadly tropical disease. Globally 2 lakh deaths are caused by this disease.

It was discovered by a German scientist and surgeon Theodore Bilharz. He identified the Schistosoma hematobium in 1851.

Synonym: snail fever, bilharzia

Causative organism

Intestinal schistosomiasis is caused by organism belonging to the genus

Schistosoma. They are S.mansoni, S.japonicum, S.mekongi, S.guineesis and S.intercalatum.

Urogenital schistosomiasis is caused by S.hematobium.

Infection and transmission

The complete lifecycle of the schistosome parasite occurs both in humans and water sources.

The freshwater snails release the larval forms of the parasite Schistosomia. When people come in contact with the infested water, these parasite penetrate into the skin. The transmission occurs when the excreta of the schistosomiasis infected person contaminate the fresh water sources. The parasite eggs are released and these eggs hatch in water sources.

In humans these larvae develops into adults forms. After penetrating to human skin through lung capillaries they migrate to blood vessels. They develop into male and female parasitic worms. These adults live in bloodstreams and blood vessels, the mating occurs in the portal blood vessels and after mating the females release their eggs to the bloodstream. Some of these eggs are excreted out by the infected person to continue the lifecycle by hatching and then continuing lifecycle. The rest of the eggs are entrapped in human tissues which in turn leads to immune reactions and subsequent damage to the organs.

Characteristics of schistosomiasis

Most of the characteristic features of schistosomiasis occurs due to exaggerated immune reactions in the human hosts against the Schistosoma eggs. These eggs release certain antigens and these antigens stimulate immunological reactions or granulomatous reactions involving immune modulators like eosinophils, macrophages and T cells.

The signs and symptoms may vary and depend upon the number and location of the eggs trapped. Initial reactions are reversible, like fever, body aches and malaise.

In later stages the tissue damage may lead to formation of regenerative nodules, collagen deposition leading to fibrosis like in case of liver cirrhosis.

Eggs also accumulate in the brain, skin, muscle, eyes and in the adrenals. In female the penetration of the eggs results in the formation of granulomas in uterus, ovaries and fallopian tube.

Diagnosis

Identification of eggs in stools and urine. It can be quantified using Kato-Katz technique, enzyme linked immunosorbent assay (ELISA) and alkaline phosphatase immunoassay.

All specimens are tested using mansoni microsomal antigen. It has a good sensitivity to infections.

Prevention

Avoid drinking contaminated water

Avoid coming in contact with contaminated water in areas that are prone to schistosomiasis.

Control of snails

Improved sanitation

Treatment

Two drugs are predominantly used in treatment are oxamniquine and praziquantel. The first line drug used is praziquantel. But its action is limited only to adult worms. Praziquantel disintegrate the worm and this in turn causes exaggerated response increasing the symptoms. This can sometime leads to sudden damage to organs.